High pigment content in the stroma blocks light from passing through the whole iris to the retina, restricting it to the pupil.(eye, human."Encyclopædia Britannica)<quoted text>
Since it is the size of the pupil that determines the amount of light entering the eyeball, I don't see what difference the color of the iris would make.
I didn't know either, and it took like 30 seconds to find out that the color of the stroma, the colored portion of the iris, does make a difference in the amount of light that hits the pupil and makes it through. Interesting, and I learned something -- but I suggest you might spend a minute of two checking.